1. Field of the Invention
The present invention generally relates to methods for forming plastic containers. More specifically, the present invention relates to a method for forming a wide-mouth, heat-set plastic container having pinch-grips and a high push-up base.
2. Description of the Prior Art
As containers made of polyethylene terephthalate (PET), or other plastic resins which are capable of being used in hot-fill applications, become more widespread, there is a need to develop these hot-fill containers so as to be suitable for an ever wider variety of product applications.
In general, heat-set or hot-fill containers are those plastic containers capable of receiving a product therein while the product is at an elevated temperature, without any resulting deformation in the container. Containers of this variety are used in those situations where the product needs to be sterilized, pasteurized or otherwise heat treated prior to filling. Upon the introduction of the hot product into the container, if the container is not of a hot-fill variety, stresses in the material forming the container will cause the container to deform into an unacceptable end product. To be considered a hot-fill container, containers must be capable of withstanding filling temperatures of at least 150.degree. F. and more typically 160.degree.-180.degree. F.
In forming a hot-fill container, PET or another suitable plastic resin is initially formed into a preform. This is most often done by an injecting molding method. Preforms all have a protypical structure which includes a mouth and a generally tubular body that terminates in a closed, typically rounded, end. Prior to being formed into containers, preforms in a softened state are transferred into a mold cavity configured in the shape of the desired container. Once in the mold cavity, the preforms are blow molded or stretch-blow molded into the desired container.
During the blow molding process, the plastic material is stretched and expanded so as to introduce an orientation (on the molecular level) into the material. The amount and location of orientation imparts various mechanical properties to the container. Generally, the higher the orientation, the less the container is capable of withstanding hot-fill temperatures. To increase the hot-fill capabilities of these oriented containers, the containers must be subsequently heat treated. The heat treatment, which can be one of several well known methods, increases the crystallinity of the material forming the container and this results in an increase in the container's thermal capabilities.
As hot-fill containers have evolved, various features have been found to increase the performance capabilities of the containers while other features have been found to enhance the usability of the containers. For example, from the performance side, hot-fill containers having deep or high push-up bases into the container cavity have been found to exhibit good mechanical and thermal properties in the base region. Specifically, the high push-up base helps to reduce the bottom roll-out which can occur after hot-filling and capping of the container. As the phrase is used herein, "high push-up base" is meant to include a base which has a domed portion that extends inwardly into the container to a height, generally measured on the exterior of the container from the contact ring of the base to the apex of the dome on the interior of the containers cavity, greater than approximately 3/8 inch and more typically 1/2 inch.
Another example of desired features in a hot-fill container are pinch-grips in the container's sidewall for easy grasping of the container.
Another desired feature for a hot-fill container is the incorporation of a large or wide-mouth into the container. Wide-mouth containers enable use of a spoon or other utensil to remove products, such as applesauce, from the container. As used herein, a wide-mouth container is generally defined as a container whose mouth has an outer diameter which is greater than approximately one-third of the outer diameter of the finished container or a mouth whose outer diameter is greater than approximately 1.5 inches.
While seen individually, the above features have not been heretofore incorporated together. As such, the novel container of the present invention may be referred to as a wide-mouth, high push-up, pinch-grip, heat-set container.
In attempting to blow molding of the aforementioned novel container, processing difficulties were encountered. These difficulties were of such a nature that no acceptable containers were formed until an equally novel method of molding the containers was developed by the present inventors.
The difficulty encountered in forming the containers was unexpectedly the result of the combination of the wide-mouth, wide body of the preform, a high push-up and the pinch-grips in the resultant container's sidewalls. The substantial indentations defining the pinch-grips in the mold were found to prevent the full formation of the base and chime areas, immediately below the pinch-grips, when a high push-up base was incorporated. It has been determined that this deficiency results from the plastic material expanding radially outward and axially downward to a point where it engages the indentations of the mold that define the pinch-grips. This contact occurs relatively early in the molding process and well before the material has been fully expanded downward onto the base and into the chime region. Once the material contacts the indentations, the material at least partially freezes and is thus restricted from being blown further down onto the base and into the chime region of the container. In addition to contact with the indentations of the mold, the material contacts the high push-up of the base mold before full expansion. This further restricts the material from being fully formed into the chime region. With less material being available for forming the base and chime regions, an incomplete or a non-uniform base was found to result. This leads to a functionally and aesthetically unacceptable container.
If a flat or a "low" push-up base is used, the inventors have found that the above difficulties mentioned above are not encountered. This is believed not to occur because the material does not contact the base mold until later in the molding process. However, as indicated earlier, a container with no or a low push-up base does not have optimum performance characteristics needed for hot-fill applications.
In view of the foregoing, it should be apparent that there exists a need for an improved wide-mouth container having pinch-grips and which is suitable for hot-fill applications. Equally, a need exists for a method of making such a container.
It is therefore a primary object of this invention to fulfill that need by providing a wide-mouth, high push-up, pinch-grip, heat-set container and a method for making such a container.
A further object of this invention is to provide a manufacturing method wherein such containers have a uniform and fully developed base.